This application claims the priority of German Patent Application Serial No. 101 33 654.3, filed Jul. 11, 2001, the subject matter of which is incorporated herein by reference.
The present invention relates, in general, to an electric machine, and more particularly to an electric synchronous machine with a rotor with permanent magnets arranged in flux concentration.
Electric synchronous machines excited by permanent magnets have certain advantages over electrically excited synchronous machines. In particular, simple synchronous machines with a small pole pitch and a small iron mass can be designed that are powered via a rectifier. Such machines operate with a higher efficiency than electrically excited synchronous machines. Permanent magnets can have a high energy density, i.e., with a large product (flux densityxc3x97field strength), and hence are superior to electromagnets, which produce a smaller energy density. It is known in the art that permanent magnets can be arranged with a mutual spacing that directly matches the effective gap, also referred to as a xe2x80x9cflatxe2x80x9d arrangement, but also in a so-called xe2x80x9cflux concentrationxe2x80x9d arrangement.
When the permanent magnets are arranged so as to directly correspond to the effective gap, the flux density of the magnets is approximately identical to the flux density in the effective gap. This is true at least as long as of the effective gap is small in relation to the height of the magnet. With the flux concentration arrangement, the flux densities in the effective gap are greater than those inside the magnet. This is achieved by arranging the magnets across a large area. When viewed in cross section, the area of the magnet is greater than the pole area in the effective gap. Accordingly, the flux density in the pole region is greater than the flux density inside the magnet. It is generally known that the motor torque is proportional to the flux density in the air gap.
It would therefore be desirable and advantageous to provide an improved electric synchronous machine, which produces a greater torque with higher efficiency than conventional synchronous machines.
According to an aspect of the invention, an electric synchronous machine has a stator and a rotor, wherein the stator may have a three-phase AC winding with an average coil width xcfx84sp and the rotor has permanent magnets, wherein the rotor has a pole pair number of 2p with a pole pitch width xcfx84p, with the ratio xcfx84sp/xcfx84p being xe2x89xa72.5. The electric machine can be an inner rotor machine, an outer rotor machine or a linear machine.
The magnetic flux density in the stator iron decreases when the ratio of coil width to pole pitch width (xcfx84sp/xcfx84p) increases. As a result of the reduced flux density in the stator iron, the radial height of the stator iron can be reduced, thereby gaining installation space.
In a configuration with a rotor with permanent magnets in flux concentration arrangement, the magnetic flux density in the air gap can be significantly greater than one Tesla, without causing the stator iron to saturate. Therefore, an electric machine operating in flux concentration can attain a magnetic flux density in the air gap that is approximately twice the flux density of conventional electric synchronous machines.
The flux concentration arrangement advantageously increases the maximum continuous torque that can be produced at a predetermined temperature, sometimes also referred to as xe2x80x9cthermal torquexe2x80x9d, as well as the maximum torque and the overall efficiency over conventional synchronous machines. Moreover, the load capacity of the machine (Nm/kg) also increases by approximately a factor of two compared to conventional electric synchronous machines. The acceleration also increases due to the smaller inertial mass.